1. Field of the Invention
The present invention relates to an apparatus for and a method of fixing a toner image to a recording medium by fusing and pressing the toner image to the recording medium in an image forming system such as a copying machine, a printer, a facsimile machine, etc.
2. Description of the Related Art
FIG. 14 of the accompanying drawings shows a conventional recent toner image fixing apparatus for use in electrophotographic machines. As shown in FIG. 14, the toner image fixing apparatus has a belt fixing system comprising a fixing roller R1, a heating and tensioning roller R3, an endless fixing belt B trained around the rollers R1, R3, and a pressing roller R2 disposed below and pressed against the fixing roller R1 with the fixing belt B interposed therebetween. When a recording medium D in the form of a sheet with an unfixed toner image carried thereon is fed into the toner image fixing apparatus by a sheet feeder, the recording medium D is reheated by the heating and tensioning roller R3, and then the toner image is fixed to the recording medium D by the fixing belt B in a nipping region between the rollers R1, R2. Since the recording medium D is preheated, the nipping region may be set to a relatively low temperature. The fixing belt B is of such a small heat capacity that when the recording medium D passes through the nipping region, the temperature of the fixing belt B is quickly lowered to increase the coherent ability of the toner which is separated from the fixing belt B at the outlet of the nipping region, for thereby allowing the toner to be easily separated from the fixing belt B. Even if the fixing belt B is free of oil or coated with a small amount of oil, a clear fixed toner image can be produced on the recording medium D without offsets. The toner image fixing apparatus shown in FIG. 14 is thus capable of solving the problems of toner separation and oil coating, which have not been eliminated by other toner image fixing apparatus using only a heating roller.
The conventional toner image fixing apparatus shown in FIG. 14 will be described in greater detail. The pressing roller R2 is positioned directly beneath the fixing roller R1, and the heating and tensioning roller R3 is disposed upstream of the fixing roller R1 with respect the direction in which the recording medium D is fed into the toner image fixing apparatus along the fixing belt B that is trained around the rollers R1, R3.
The toner image fixing apparatus also has an oil coating roller R4 disposed above an upper run of the fixing belt B. A guide plate G for supporting the recording medium D is disposed below a lower run of the fixing belt B, and a gap between the guide plate G and the lower run of the fixing belt B serves as a preheating passage P for preheating the recording medium D when the recording medium D travels below the heating and tensioning roller R3 toward the nipping region.
The fixing belt B is tensioned to a desired tension level when the heating and tensioning roller R3 is pushed away from the fixing roller R1 by a pressing lever U. The fixing belt B is actuated by the fixing roller R1 which is coupled to an actuator. Since the fixing belt B is appropriately tensioned, it can stably rotate around the rollers R1, R3 without undesirable slippage and sagging.
A heater H is housed in the heating and tensioning roller R3. The heating and tensioning roller R3 is associated with a thermistor S for measuring the temperature of the surface of the heating and tensioning roller R3. The fixing belt B on the heating and tensioning roller R3 has a sheet-contact area which is contacted by the recording medium D that is fed from the sheet feeder and a non-sheet-contact area which is not contacted by the recording medium D that is fed from the sheet feeder. The thermistor S is kept out of contact with the sheet-contact area of the fixing belt B on the heating and tensioning roller R3, but held in contact with the non-sheet-contact area of the fixing belt B on the heating and tensioning roller R3.
During the fixing process, based on a signal from the thermistor S, a controller (not shown) connected to the thermistor S controls the amount of heat generated by the heating and tensioning roller R3 so that the temperature of the surface of the heating and tensioning roller R3 will be kept at a preset level.
The temperature of the fixing belt B on the fixing roller R1 varies depending on the period of time in which the fixing belt B has rotated, and is not constant when the recording medium D passes through the nipping region. If the period of time in which the fixing belt B has rotated is short, then the temperature of the fixing belt B on the fixing roller R1 is low. In order to increase the temperature of the fixing belt B on the fixing roller R1, it is necessary to increase a temperature setting for the heating and tensioning roller R3 for thereby bringing the temperature of the fixing belt B on the fixing roller R1 into a toner image fixing temperature range at all times.
If the toner image fixing apparatus shown in FIG. 14 is used to produce successive full-color copies, since the period of time in which the fixing belt B has rotated increases, the temperature of the fixing belt B on the fixing roller R1 also increases, and so does the temperature of the outlet of the nipping region. FIG. 15 of the accompanying drawings shows temperature characteristics of the toner image fixing apparatus shown in FIG. 14.
In FIG. 15, the horizontal axis represents the period of time in which the fixing belt B has rotated, and the vertical axis represents the temperature of the fixing belt B on the rollers R1, R3. First, a temperature characteristic of the toner image fixing apparatus at the time the amount of heat radiated by the heater H is controlled in order to equalize the temperature of the fixing belt B on the heating and tensioning roller R3 to a preset temperature T2 will be described below. A solid-line wavy curve W1 represents the temperature of the fixing belt B on the heating and tensioning roller R3, and a solid-line curve C1 represents the temperature of the fixing belt B on the fixing roller R1.
After a standby period, as the period of time in which the fixing belt B has rotated increases, the temperature of the fixing belt B on the fixing roller R1 increases. When the temperature of the fixing belt B on the fixing roller R1 exceeds an upper limit temperature T1 of a toner image fixing temperature range, the possibility of hot sheet offsets, i.e., sheet offsets at high temperatures, or sheet jams increases. When the temperature of the fixing belt B on the fixing roller R1 becomes lower than a lower limit temperature T1' of the toner image fixing temperature range, the possibility of cold sheet offsets, i.e., sheet offsets at low temperatures, or unfixed toner regions increases. Therefore, the temperature of the fixing belt B on the fixing roller R1 should be kept in the toner image fixing temperature range which lies between the upper limit temperature T1 and the lower limit temperature T1'.
The above drawback, i.e., sheet offsets and sheet jams, can be avoided when the temperature of the fixing belt B on the heating and tensioning roller R3 is set to a temperature T2', lower than the preset temperature T2, such that the temperature of the fixing belt B on the fixing roller R1 will be equal to or below the upper limit temperature T1 at its maximum, as indicated by broken-line characteristics curves W2, C2. However, it will take a longer period of time for the temperature of the fixing belt B on the fixing roller R1 to reach the lower limit temperature T1' of the toner image fixing temperature range, with the result that a fixation readiness time, i.e., a period of time required for the toner image fixing apparatus to become ready for fixing toner images, increases from TS to TS'.
After the toner image on the recording medium D is fixed, the sheet feeder for feeding the recording medium D into the toner image fixing apparatus is deactivated, the operation of the fixing belt B is stopped, and the heater H is de-energized, whereupon the toner image fixing apparatus enters a standby mode. Once the toner image fixing apparatus enters the standby mode, the surface temperatures of the fixing belt B and the fixing roller R1 fall gradually. If the standby mode continues for a long period of time, then the fixing belt B and the fixing roller R1 become so cold that when a fixing process is started again, it will take a long period of time before the fixing roller R1 is heated to the toner image fixing temperature range. As a result, the operator has to wait a long period of time before the toner image fixing apparatus is operational again.
To alleviate the above deficiency, there has been proposed a priority control process which employs an auxiliary thermistor (not shown) for measuring the temperature of the surface of the fixing roller R1. According to the proposed priority control process, as shown in FIG. 16 of the accompanying drawings, until the surface temperature of the fixing roller R1 rises nearly to the toner image fixing temperature range, the amount of heat radiated by the heater H is controlled on the basis of the surface temperature of the heating and tensioning roller R3 as measured by the thermistor S. When the surface temperature of the fixing roller R1 increases beyond the toner image fixing temperature range, the amount of heat radiated by the heater H is controlled on the basis of the surface temperature of the fixing roller R1 as measured by the auxiliary thermistor. The priority control process is effective to prevent sheet offsets and sheet jams from occurring, and also to shorten the period of time required to heat the fixing roller R1 to the toner image fixing temperature range after the standby mode.
Image forming systems such as electronic copying machines, electronic printers, etc. which incorporate the above toner image fixing apparatus are required in recent years to operate at a higher speed to meet demands for a higher sheet feed speed, i.e., an increased number of sheets fed per unit time through the toner image fixing apparatus. To meet such requirements, the fixing belt B needs to run at a higher speed, which results in a reduction in the amount of heat that is transferred per unit time from the heating and tensioning roller R3 to the fixing belt B.
As described above, the thermistor S is held in contact with the non-sheet-contact area of the fixing belt B on the heating and tensioning roller R3. When sheets, e.g., recording mediums D, are successively fed into the toner image fixing apparatus, since the non-sheet-contact area of the fixing belt B on the heating and tensioning roller R3 is not contacted by the sheets, the heat in the non-sheet-contact area of the fixing belt B is not dissipated, but stored therein, so that the temperature as measured by the thermistor S increases to a level beyond a heater control switching point shown in FIG. 16. When the heater control switching point is reached while successive sheets are being fed into the toner image fixing apparatus, the controlling of the amount of heat radiated by the heater H on the basis of the surface temperature of the fixing roller R1 as measured by the auxiliary thermistor switches to the controlling of the amount of heat radiated by the heater H on the basis of the surface temperature of the heating and tensioning roller R3 as measured by the thermistor S.
As a consequence, though the amount of heat radiated by the heater H is kept at a constant level based on the temperature measured by the thermistor S, the heat of the fixing roller R1 is greatly absorbed by the sheets that are being fed successively at a high speed. Therefore, as shown in FIG. 17 of the accompanying drawings, the surface temperature of the fixing roller R1 gradually falls. According to the priority control process, since the surface temperature of the fixing roller R1 gradually falls while sheets are being fed successively at a high speed, toner images may not be fixed to the sheets with good toner image fixability.
It has been proposed to incorporate another heater in the pressing roller R2 to meet the requirements for the toner image fixing apparatus to operate at a higher speed.
When small-size sheets or recording mediums D are successively fed into the toner image fixing apparatus, those sheets are not brought into contact with a non-sheet-contact area of the heating and tensioning roller R3 which is associated with the thermistor S. Therefore, the non-sheet-contact area of the heating and tensioning roller R3 stores a large amount of heat, and hence its temperature rises excessively, as shown in FIG. 15.
When the temperature non-sheet-contact area of the heating and tensioning roller R3 increases excessively, the surface temperature of the fixing roller R1 also increases excessively. The fixing roller R1 thus tends to deteriorate soon, have a shortened service life, cause an increased energy loss, and pose safety problems.